In cellular communications networks such as 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) networks, there are two types of deployments, namely, a homogenous network and a heterogeneous network. A homogeneous network utilizes a single layer, or tier, of radio network nodes. In one particular example, all radio network nodes in a homogeneous network are High Power Nodes (HPNs) such as wide area base stations serving macro cells. As another example, all radio network nodes in a homogeneous network are Low Power Nodes (LPNs), e.g., local area base stations serving pico cells. When there are similar load levels in the different cells of a homogeneous network, a wireless device, which is sometimes referred to as a User Equipment device (UE) or terminal, typically receives equally strong signals from a serving or measured cell and from a closest neighboring cell(s), especially when the UE is located in the in the cell border region. Therefore, in a homogeneous network, resource partitioning between serving and neighboring cells for the purpose of inter-cell interference mitigation is not as critical as in heterogeneous networks.
A heterogeneous network includes two or more layers of radio network nodes. In particular, each layer of the heterogeneous network is served by one type, or class, of Base Stations (BSs). In other words, a heterogeneous network includes a set of HPNs (e.g., a set of high power or macro BSs) and a set of LPNs (e.g., a set of low power or medium range, local area, or home BSs) in the same geographical region. A BS power class is defined in terms of maximum output power and other radio requirements (e.g., frequency error, etc.) which depend upon the maximum output power. The maximum output power, Pmax, of the BS is the mean power level per carrier measured at the antenna connector in a specified reference condition. The rated output power, PRAT, of the BSs for different BS power classes is expressed in Table 1 below.
TABLE 1Base Station rated output power in LTE (FDD and TDD)BS classPRATWide Area BS— (note)Medium Range BS<+38 dBmLocal Area BS≤+24 dBmHome BS≤+20 dBm (for one transmit antenna port)≤+17 dBm (for two transmit antenna ports)≤+14 dBm (for four transmit antenna ports)<+11 dBm (for eight transmit antenna ports)NOTE:There is no upper limit for the rated output power of the Wide Area Base Station.As stated above some of the requirements may also differ between BS classes. For example, as shown in Table 2 below, the frequency error is worse for LPNs. The frequency error is the measure of the difference between the actual BS transmitted frequency and the assigned frequency.
TABLE 2Frequency error minimum requirement in LTE (FDD and TDD)BS classAccuracyWide Area BS±0.05 ppmMedium Range BS ±0.1 ppmLocal Area BS ±0.1 ppmHome BS±0.25 ppmA wide area BS serves a macro cell, a medium range BS serves a micro cell, a local area BS serves a pico cell, and a home BS serves a femto cell. Typically, a wide area BS is regarded as a HPN, whereas all the remaining classes of BSs can be regarded as LPNs.
In a two layer macro-pico heterogeneous network, the macro cell and pico cell layers typically include wide area BSs, which are also known as macro BSs, and local area BSs, which are also known as pico BSs, respectively. The high data rate wireless devices located close to the pico BSs (i.e., in the pico layer) can be offloaded from the macro layer to the pico layer. A more complex heterogeneous deployment may include three layers, namely, a macro layer, a micro layer that is served by medium range BSs, and a pico layer. An even more complex heterogeneous deployment may include three layers, namely, a macro layer, a pico layer, and a home or femto layer.
Heterogeneous networks, and in particular the co-channel scenario utilized by heterogeneous networks, bring more challenges in terms of managing interference. For example, inter-cell interference experienced by the UE in the downlink and by the BS in the uplink needs to be mitigated. To address this issue, Inter-Cell Interference Coordination (ICIC), Enhanced ICIC (eICIC) and Further eICIC (FeICIC) techniques have been developed in 3GPP. The eICIC and FeICIC techniques are time domain schemes in that they enable interference mitigation by virtue of resource partitioning in the time domain between the aggressor, or interfering, cell and the victim cell. This in turns partly or fully mitigates the interference towards the victim cell, or more specifically at the receiver of a victim wireless device in the victim cell.
According to the time domain eICIC or FeICIC schemes, subframe utilization across different cells is coordinated in time through backhaul signaling, which for LTE is backhaul signaling over X2 connections between BSs. Subframe utilization is expressed in terms of a time domain pattern of low interference subframes or “a low interference transmit pattern.” More specifically, these low interference transmit patterns are referred to as Almost Blank Subframe (ABS) patterns. The ABSs are configured in an aggressor cell (e.g., a macro cell) and are used to protect resources in subframes in the victim cell (e.g., a pico cell) receiving strong inter-cell interference. The serving BS signals one or more measurement patterns to inform the UE about the resources or subframes that the UE should use for performing measurements on a target victim cell (e.g., a serving pico cell and/or neighboring pico cells). These measurement patterns are more specifically referred to as time domain measurement resource restriction for the Primary Cell (PCell) and time domain measurement resource restriction for the neighbor cells. Each measurement pattern includes of bit map of subframes (e.g., 10000000) where 1 indicates a subframe that is available for measurements and 0 indicates a subframe that is not available for measurements. Typically, there are 1-2 restricted subframes per radio frame since traffic density in an LPN is much lower compared to that in a HPN. Examples of measurements are Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), Channel State Information (CSI) (e.g., Channel Quality Indication (CQI), Rank Indicator (RI), Precoding Matrix Indicator (PMI), etc.). While there can be measurement restriction patterns, there are not such patterns for restricting scheduling of a UE. As such, typically, a UE is also scheduled in restricted subframes which overlap with low interference subframes (e.g., ABS) in aggressor cells. Therefore, UEs experience better signal quality in these subframes.
In cellular networks, a wireless device is normally configured to report a CQI to a serving BS and thereby indicating a Signal-to-Interference plus Noise Ratio (SINR) observed by the wireless device in the downlink from the serving BS. Based on this CQI report, the serving BS selects a suitable Modulation and Coding Scheme (MCS) to be used when transmitting data to the wireless device in the downlink. The wireless device typically derives the CQI by first estimating the downlink channel of the serving BS and then estimating the interference and noise as the residual obtained by removing an estimated desired signal from the received signal. Interference estimation in LTE for CQI estimation is performed over a set of predefined, or configured, Resource Elements (REs). In LTE Release 8 (Rel-8) to Release 10 (Rel-10), interference measurements are expected to be done on REs carrying a Cell-Specific Reference Signal (CRS), whereas in LTE Release 11 (Rel-11) dedicated Interference Measurement Resources (IMRs) were introduced in conjunction with LTE transmission mode 10.
In 3GPP LTE networks, downlink transmissions are based on Orthogonal Frequency Division Multiplexing (OFDM) in which physical resources can be seen as a time-frequency grid of REs, where physical channels and signals are mapped to specific REs. One type of downlink physical signal refers to the CRS, which is used for demodulation of data as well as for mobility measurements and CQI estimation. The CRS is regularly transmitted by all cells, and the structure and locations of the CRS in the time-frequency grid are known after cell acquisition. The density of CRS symbols depends on the number of configured antenna ports. In LTE, a cell can be configured with 1, 2, or 4 antenna ports. The locations of the CRS symbols can be shifted in the frequency domain where the particular shift is given by the physical layer cell identity. In deployments with more than one antenna port, three frequency shifts can be considered. In LTE, a downlink subframe can be configured as “Multicast-Broadcast Single-Frequency Network (MBSFN)” which means that CRS is not present in the data region of the subframe. As CRSs are common to all wireless devices in a cell, CRSs are not precoded and are always transmitted with full power.
For LTE transmission modes 1 to 9, interference measurements as part of deriving CQI are expected to be done on REs carrying CRS of the serving cell. These interference measurements are then used to predict the interference on REs carrying data. The accuracy with which the interference measurements on the REs carrying CRS reflects the interference on the data depends on both the CRS locations and the traffic load of the interfering neighboring cells, i.e. the aggressor cells. In time synchronized LTE networks, CRS transmissions in aggressor cells may either interfere with serving cell REs carrying CRS or REs carrying data, depending on CRS frequency shifts among the cells. Thus, this implies CRS-to-CRS collisions across cells when a non-shifted configuration is used in all cells of a synchronized network. On the other hand, CRS-to-CRS collisions across cells can be partially avoided if shifted CRS configurations are used among cells. However, CRS-to-CRS collisions can in general not be completely avoided with only three frequency shifts, and sometimes non-shifted configurations can be preferred from a user throughput perspective in low-to-medium loaded traffic scenarios.
In a time synchronized network scenario with two dominating aggressor cells, the inter-cell interference on the CRS REs in the cases of non-shifted and shifted CRS configurations can be expressed as:
                    I        CRS            =                                                  I              CRS                              NC                ⁢                                                                  ⁢                1                                      +                          I              CRS                              NC                ⁢                                                                  ⁢                2                                                          ︸                          LOAD              ⁢              _              ⁢              INDEPENDENT                                      ⁢                                  ⁢                  (                      non            ⁢                          -                        ⁢            shifted                    )                      ,    and                      I        CRS            =                                                  I              DATA                              NC                ⁢                                                                  ⁢                1                                      +                          I              DATA                              NC                ⁢                                                                  ⁢                2                                                          ︸                          LOAD              ⁢              _              ⁢              DEPENDENT                                      ⁢                                  ⁢                  (          shifted          )                      ,  where ICRSNCx and IDATANCx, for x=1,2, represent the (averaged) interference caused by Neighbor Cell (NC) CRS and data transmissions, respectively. In the non-shifted scenario, the measured interference refers to only the CRS transmissions of the neighbor cells. Since CRSs are transmitted with full power, a wireless device will measure high interference independently of the traffic load in the neighbor cells in the non-shifted scenario. Thus, such interference measurements can only be representative in scenarios where the aggressor cells are highly loaded. In contrast, in the shifted scenario, the measured interference refers to data transmissions of the neighbor cells, and the interference level observed by a wireless device would then depend on the traffic load in the aggressor cells. As the purpose of the interference measurements is to predict the interference level on data, it can be noticed that the non-shifted case will typically over-estimate the interference level whereas the shifted case will underestimate the interference level as the CRS based interference measurements would not capture the impact of the CRS transmissions of the aggressor cells as illustrated by following expressions:IDATA=IDATANC1+IDATANC2 (non-shifted), andIDATA=ICRSNC1+ICRSNC2+IDATANC1+IDATANC2 (shifted).
From the expression IDATA (shifted) it is evident that there will be interference on the data even when there is no scheduled downlink traffic in the aggressor cells, i.e. IDATANCx=0. However, the CRS represents only a fraction of the REs within a resource block (roughly 10%) so the relative impact of the CRS interference on the total interference depends on the traffic load in the aggressor cells. As the traffic load in the aggressor cells increases, the impact of the CRS on the total interference decreases. More specifically, as the traffic load increases, the CRS represents a smaller fraction of the total interference and, as such, the impact of the CRS on the total interference decreases. As in the non-shifted case, the interference measurements will reflect the interference level on data most accurately when the aggressor cells are highly loaded.
In LTE Rel-11 under the FeICIC Work Item, support for Interference Cancellation (IC) by the wireless device on CRS REs (IC-CRS) was introduced. The wireless device has the capability of removing a number of interfering (or aggressor) cells on those REs. The amount of aggressor cells that can be removed is up to two, but in principle it can be any positive number upper bounded by the number of discovered interferers. By applying IC-CRS, channel estimation performance can be further improved due to less noisy signal samples in the non-shifted CRS case. In addition, CRS inter-cell interference on data REs can be further reduced in case of shifted CRS configuration. Furthermore, in order to simplify the IC-CRS implementation on the wireless device side, network assisted Radio Resource Control (RRC) signaling was introduced in LTE Rel-11. With this signaling, the serving cell informs the wireless device of the physical layer cell identities and corresponding number of antenna ports of up to eight potential aggressor cells. When the wireless device has acquired this information, the wireless device knows the locations of the CRS in potential aggressor cells without autonomously detecting these locations.
In an LTE Rel-11 co-channel heterogeneous network deployment, a large Cell Range Expansion (CRE) of up to 9 Decibels (dB) is supported. When a wireless device is in the CRE region of an LPN (e.g., a pico, micro, or femto/home BS), the received signal at the wireless device can be interfered by up to two strong macro aggressor cells. Therefore, in this scenario, the received SINR (aka Synchronization Channel (SCH) Ês/lot or CRS Ês/lot) at the wireless device served by the LPN when located in the CRE region of the serving cell can be very low, e.g. down to −11 dB. The SCH herein includes one or more of Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS).
In order to correctly detect received signals, the wireless device in the CRE region has to cancel interference on certain physical signals (e.g., CRS, PSS/SSS) and certain physical channels (e.g., Physical Broadcast Channel (PBCH)). To facilitate interference cancellation or mitigation of these physical signals and/or physical channels at the wireless device, a radio network node can assist the wireless device by providing a list of assistance data as specified in 3GPP Technical Specification (TS) 36.331 for Release 11:
 NeighCellsCRS-Info-r11 ::=CHOICE {   release NULL,   setup CRS-AssistanceInfoList-r11 }CRS-AssistanceInfoList-r11 ::=SEQUENCE (SIZE (1. . maxCellReport)) OF CRS-AssistanceInfo  CRS-AssistanceInfo ::= SEQUENCE {   physCellID-r11  PhysCellID   antennaPortsCount-r11  ENUMERATED {an1, an2, an4, spare1},   mbsfn-SubframeConfigList-r11  MBSFN-SubframeConfigList } -- AN1STOPRadioResourceConfigDedicated field descriptionsneighCellsCRSInfoThis field contains assistance information, concerning the primary frequency, used by the UE to mitigate interference from CRS while performing RRM/RLM/CSI measurement or data demodulation. The UE forwards the received CRS assistance information to lower layers.When the received CRS assistance information is for a cell with CRS colliding with that of the CRS of the cell to measure, the UE may use the CRS assistance information to mitigate CRS interference (as specified in [FFS]) on the subframes indicated by measSubframePatternPCell, measSubframePatternConfigNeigh and csi-MeasSubframeSet1. Furthermore, the UE may use CRS assistance information to mitigate CRS interference from the cells in the IE for the demodulation purpose as specified in [FFS].According to the above Information Element (IE), the CRS assistance data contains list of aggressor cells, their antenna port information, and their MBSFN configuration.
It has also been specified in 3GPP TS 36.133 V11.2.0 that the wireless device shall meet the measurement requirements when the wireless device is provided with CRS assistance information, which is valid over the measurement period. Therefore, the reception of the CRS assistance data at the wireless device is used by the wireless device to perform the IC on, e.g., CRS, PSS/SSS, etc. However, in a heterogeneous network deployment, the wireless device typically applies IC on restricted subframes indicated in measurement patterns, which are signaled to the wireless device by the serving radio node via RRC protocol as described above.
RSRP and RSRQ are two existing radio measurements performed by the wireless device. RSRP and RSRQ measurements are used for at least Radio Resource Management (RRM) purposes such as, e.g., mobility, which includes mobility in the RRC connected state as well as mobility in the RRC idle state. RSRP and RSRQ measurements are also used for other purposes such as, e.g., enhanced cell Identity (ID) positioning, Minimization of Drive Test (MDT), etc.
RSRP and RSRQ measurements can be absolute or relative. An absolute measurement is performed on signals from one cell, e.g. a serving cell or a neighboring cell. A relative measurement is the relative difference between the measurement performed on one cell and on another cell, e.g. between a serving cell measurement and a neighboring cell measurement.
CSI measurements performed by the wireless device on the serving cell are used by the network for scheduling, link adaptation, etc. Examples of CSI measurements are CQI, PMI, RI, etc.
The radio measurements performed by the wireless device are used by the wireless device for one or more radio operational tasks. One example of such a task is reporting the measurements to the network, which in turn may use them for various tasks. For example, when in the RRC connected state, the wireless device reports radio measurements to the serving BS of the wireless device. In response to the reported measurements, the serving BS takes certain decisions, e.g. it may send a mobility command to the wireless device for the purpose of a cell change. Examples of cell change are a handover, an RRC connection re-establishment, an RRC connection release with redirection, a PCell change in Carrier Aggregation (CA), Primary Component Carrier (PCC) change in PCC, etc. In the RRC idle or low activity state, one example of a cell change is a cell reselection. In another example, the wireless device may itself use the radio measurements for performing tasks, e.g. cell selection, cell reselection, etc.
In order to support different functions such as mobility (e.g., cell selection, handover, etc.), positioning a wireless device, link adaption, scheduling, load balancing, admission control, interference management, interference mitigation, etc., a radio network node (e.g., a BS) also performs radio measurements on signals transmitted and/or received by the radio network node. Examples of such measurements are Signal-to-Noise Ratio (SNR), SINR, Received Interference Power (RIP), Block Error Ratio (BLER), propagation delay between a wireless device and itself, transmit carrier power, transmit power of specific signals (e.g., Transmit (Tx) power of reference signals), positioning measurements, etc.
In a multi-carrier or CA system, a wireless device is served by multiple Component Carriers (CCs), which are also sometimes referred to as cells or serving cells. The term CA is also called (e.g., interchangeably called) “multi-carrier system,” “multi-cell operation,” “multi-carrier operation,” or “multi-carrier” transmission and/or reception. CA is used for transmission of signaling and data in the uplink and downlink directions. One of the CCs is a PCC, which may also be referred to simply as a primary carrier or even an anchor carrier. The remaining CC(s) are referred to as a Secondary Component Carrier(s) (SCC(s)) or simply a secondary carrier(s) or even a supplementary carrier(s). Generally, the PCC carries essential wireless device specific signaling. The PCC, which is also known as the PCell, exists in both uplink and downlink directions in CA. In case there is single uplink CC, the PCell is obviously on that CC. The network may assign different PCCs to different wireless devices operating in the same sector or cell.
Therefore, in CA, the wireless device has more than one serving cell in the downlink and/or in the uplink: one serving PCell and one or more serving Secondary Cells (SCells) operating on the PCC and SCC(s), respectively. The PCell is interchangeably referred to as a Primary Serving Cell (PSC). Similarly, the SCell(s) is(are) interchangeably referred to as a Secondary Serving Cell(s) (SSC(s)). Regardless of the terminology, the PCell and SCell(s) enable the wireless device to receive and/or transmit data. More specifically, the PCell and SCell(s) exist in the downlink and uplink for the reception and transmission of data by the wireless device. The remaining non-serving cells on the PCC and SCC are called neighbor cells.
The CCs belonging to the CA scheme may belong to the same frequency band (for intra-band CA), different frequency bands (for inter-band CA), or any combination thereof (e.g., two CCs in band A and one CC in band B). Inter-band CA including carriers distributed over two bands is also referred to as Dual-Band-Dual-Carrier-High Speed Downlink Packet Access (DB-DC-HSDPA) in HSPA or inter-band CA in LTE. Furthermore, the CCs in intra-band CA may be adjacent or non-adjacent in frequency domain. The non-adjacent case is referred to as intra-band non-adjacent CA). A hybrid CA including intra-band adjacent, intra-band non-adjacent, and inter-band is also possible. Using CA between carriers of different technologies is also referred to as “multi-Radio Access Technology (RAT) CA,” “multi-RAT-multi-carrier system,” or simply “inter-RAT CA.” For example, the carriers from Wideband Code Division Multiple Access (WCDMA) and LTE may be aggregated. Another example is the aggregation of LTE and Code Division Multiple Access (CDMA) 2000 carriers. Yet another example is the aggregation of LTE Frequency Division Duplexing (FDD) and LTE Time Division Duplexing (TDD) carriers. For the sake of clarity, CA within the same RAT can be regarded as “intra-RAT” or simply “single RAT” CA.
Multi-carrier operation may also be used in conjunction with multi-antenna transmission. For example, signals on each CC may be transmitted by the BS to the wireless device over two or more antennas. Further, the CCs used for CA may or may not be co-located in the same site or BS or radio network node (e.g., relay, mobile relay, etc.). For instance, the CCs may originate (i.e., be transmitted/received) at different locations (e.g., from non-co-located BSs, or from a BS and a Remote Radio Head (RRH) or Remote Radio Unit (RRU)). Examples of combined CA and multi-point communication include Distributed Antenna System (DAS), RRH, RRU, Coordinated Multi-Point (CoMP), multi-point transmission/reception, etc.
Several positioning methods for determining the location of a target device, which can be a wireless device, a mobile relay, a Personal Digital Assistant (PDA), or the like may be used. These methods include:                Satellite based methods: Satellite based methods use Assisted Global Navigation Satellite System (A-GNSS) (e.g., Assisted Global Positioning System (A-GPS)) measurements for determining position of a target device.        Observed Time Difference of Arrival (OTDOA): OTDOA methods use Reference Signal Time Difference (RSTD) measurements for the target device to determine the position of the device in LTE.        Uplink Time Difference of Arrival (UTDOA): UTDOA uses measurements done at a Location Management Unit (LMU) to determine the position of a target device.        Enhanced cell ID: Enhanced cell ID based methods use one or more of UE Receive (Rx)-Tx time difference, BS Rx-Tx time difference, LTE RSRP/RSRQ, High Speed Packet Access (HSPA) Common Pilot Channel (CPICH) measurements, Angle of Arrival (AoA), etc. to determine UE position. Fingerprinting is considered to be one type of enhanced cell ID method.        Hybrid methods: Hybrid methods use measurements from more than one method for determining UE position.In LTE, the positioning node, which is also known as an Evolved Serving Mobile Location Centre (E-SMLC) or location server, configures the wireless device, BS, or LMU to perform one or more positioning measurements. The positioning measurements are used by the wireless device or the positioning node to determine the location of the wireless device. The positioning node communicates with wireless device and the BS in LTE using LTE Positioning Protocol (LPP) and LPP A (LPPa) protocols, respectively.        